Mod

A mod (shortened from modification) is an fan-made modification or an expansion of an existing game.

Mod types

Partial conversion mods - these mods retain most of the existing assets, but gain new units, factions and sometimes mechanics.

Total conversion mods - the entire game theme is overhauled, factions and units are most often completely different than found in the game that is modified or the majority of their assets have been replaced or redone. Game mechanics are often added or expanded.

Graphics mods - these mods only alter the graphical appearance of selected elements of the game. They are often compatible with users who do not have them.

Unofficial patches - often done in the form of a mod, these projects do not add new assets to the game, and instead tend to fix many of the bugs left over after the original game's support ended. Some of them also change the balance to improve the game experience.